Some wireless communication systems include a transmit-receive switch, hereinafter referred to as a TR switch, to transmit and receive signals using the same antenna. Examples of such wireless communications include magnetic field based communication systems (also referred to herein as inductive communication systems) having a single inductive coil. In a transmit mode, the inductive coil is energized to transmit a signal through a resulting time-varying magnetic field. In a receive mode, the inductive coil induces a voltage when in the presence of a time-varying magnetic field that is representative of a signal contained in the field.
Preferably, the frequency of the time-varying magnetic field for a transmitted signal closely corresponds with the resonant frequency of the transmitting circuit. A strong signal is transmitted from the antenna by energizing the coil with a large current at the resonant frequency. The receiving circuit has a resonant frequency that corresponds to the frequency of the time-varying magnetic field that contains the signal to be received.
Unfortunately, the TR switch introduces a resonance frequency shift by switching the antenna between driving circuitry for a transmit mode and receiving circuitry for a receive mode. This shift is attributable to the input capacitance of a low noise amplifier (LNA), which is also referred to herein as a DC blocking capacitor to the LNA. The DC block capacitor forms part of the resonance frequency in the transmit mode and does not form part of the resonance frequency in the receive mode. Furthermore, in the transmit mode, the large current used to drive the coil shifts the value of the coil inductance in a manner that adds to the resonance frequency shift attributable to the DC blocking capacitor. The resonance frequency shift penalizes the range between the transmitter and receiver because of the reduced power transfer associated with the frequency mismatch between the transmitting and receiving devices.
There is a need in the art to provide improve wireless communication systems.